The overall goal of this application is the study of the effect of the active hormonal form of vitamin D, l,25-dihydroxyvitamin D3 (l,25D), on prostate cancer. In addition to its well known role in calcium homeostasis, l ,25D is a potent inhibitor of prostate cancer cell growth. The long-term goal of this application is to develop novel, effective and low toxicity (less hypercalcemia) treatments for prostate cancer. To accomplish this goal, two strategies will he investigated: (l) to evaluate analogs of l,25D that have increased potency at growth inhibition and decreased calcemic activity and (2) combination therapy using other agents such as the retinoids along with l,25D and/or l,25D analogs. The application has three specific aims which have been designed to build upon one another. Specific Aim 1 will investigate the growth-inhibitory effect of l,25D and analogs and combination therapies in cultured human prostate cancer cells. We will study both primary cancer cells and established cancer cell lines. 1,25D acts by binding to the vitamin D receptor (VDR), which then interacts with specific nucleotide sequences in the promoter region of 1,25D-responsive genes, vitamin D response elements (VDRE). In an effort to understand why analogs are less calcemic yet more potent antiproliferative agents, we will examine 1,25D and analog-mediated gene expression in prostate cancer cells and in COS-7 cells using different transfected VDRE-reporter gene constructs. Because differential gene activation may explain the less calcemic effect of the analogs, we will study analog-VDR interactions with various target gene VDREs. Information generated here will he useful in designing new analogs with even more potent antiproliferative and differentiating activity. Specific Aim 2 will apply knowledge gained in Aim l to an in vivo model of prostate cancer. Immunodeficient (nude) mice xenografted with human prostate cancer cells will be treated with l,25D and analogs alone or in drug combinations that have been shown to he more effective than l,25D in Aim l. Building upon information in the first two specific aims, Specific Aim 3 describes a clinical trial using analog or combination therapy protocols derived from work in Aims 1 and 2 to treat prostate cancer in patients. These clinical studies will use prostate specific antigen (PSA) as a tumor marker and evaluate efficacy of l,25D analogs and/or drug combinations by changes in the doubling time of PSA. It is hoped that the information gained will lead to more effective and less toxic treatments for prostate cancer.